Soil organic carbon changes under selected agroforestry cocoa systems in Ghana

Given the pressing issue of climate change, there is a clear need for long-term and reliable soil organic carbon (SOC) data. This data should be complemented by insights into agricultural management practices, with a particular emphasis on agroforestry systems. Unfortunately, such comprehensive data remains in short supply. We report SOC and bulk density changes in the soil (<= 60 cm depth) of Ghanaian agroforestry cocoa systems sampled along a chronosequence of farm fields established 3, 15, 18, 30 and 45 years after forest conversion. Additionally, we assessed changes in SOC and bulk density from agroforestry cocoa systems fifteen years after similar data was collected from the same fields. The 45-year-old plot significantly had higher (p < 0.05) SOC concentration, comparable to the pre-conversion forest. Cocoa trees aged 18 and above had higher SOC concentrations than the forest at the 20-60 cm depth. The SOC stocks initially declined by 18% (12 Mg ha(-1)) after forest conversion in the three-year-old cocoa farm but increased considerably thereafter to 121 Mg C ha(-1) after 45 years of cocoa production. The 45-year-old cocoa plantation had more than twice the SOC stocks observed in the forest at the 20-60 cm depth. A 15-year period comparison of agroforestry cocoa systems revealed proportional increases in SOC stocks, with the exception of minor declines observed within specific age groups at the 0-10 cm depth. Mean bulk densities did not exceed the critical value of 1.6 g cm(-3) above soil conditions considered unsuitable for plant growth. Soil bulk densities generally increased with depth, and agroforestry cocoa systems had significantly (p < 0.05) denser soils compared to the forest. Our study provides compelling long-term empirical evidence emphasizing the importance of agroforestry cocoa systems on SOC storage and reinforces their role as a sustainable and climate-smart approach to agriculture.